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18-Crown-6 is used as an efficient phase transfer catalyst and as a complexing agent with a variety of small cation. It is involved in the synthesis of diaryl ethers, diaryl thioethers, and diarylamines mediated by potassium fluoride-alumina and 18-crown-6. It facilitates the solubility of potassium permanganate in benzene, which is used for oxidizing the organic compounds. It is used to accelerate various substitution reactions as well as enhances the power of nucleophiles such as potassium acetate. It is utilized in the alkylation reactions in the presence of potassium carbonate, N-alkylation of glutarimide and succinimide with dimethylcarbonate. The complex formed by its reaction with potassium cyanide acts as a catalyst in the cyanosilylation of aldehydes, ketones and quinines with trimethylsilyl cyanide (TMSCN).
Crown ether (see Appendix 2) mainly used to complex potassium ions. For a comprehensive study of solvent effects on complexation with K ions in a wide range of solvents, see: J. Org. Chem., 61, 5221 (1996).
Has been widely used in alkylation reactions with, e.g. K2CO3 as base:
Enables KH to metallate aryl-substituted methanes directly: J. Am. Chem. Soc., 99, 4457 (1977). In the presence of KOH in DME, various arylmethanes, benzyl alcohols and aldehydes are oxidized to carboxylic acids by molecular oxygen: Tetrahedron Lett., 25, 4989 (1984).
Has been used with KF to enhance the reactivity of F- as a base or a nucleophile, e.g. to promote Michael additions in aprotic solvents: J. Chem. Soc., Chem. Commun., 237 (1977), the condensation of nitromethane with aldehydes: Tetrahedron Lett., 3219 (1978), and the transesterification of diaryl to dialkyl phosphonates: Synthesis, 409, 412 (1982).
Crown ether catalysis can be used in the reaction of KCN with alkyl halides to give nitriles: Tetrahedron Lett., 71 (1975), and for the preparation of acyl cyanides from acid chlorides: Tetrahedron Lett., 21, 2959 (1980). The 1:1 complex between 18-crown-6 and KCN has been used catalytically in the cyanosilylation of aldehydes, ketones and quinones with TMSCN; for examples, see: Org. Synth. Coll., 7, 517 (1990).
Flórez, A. I. G.; Ahn, D. S.; Gewinner, S.; Schöllkopf, W.; von Helden, G. IR spectroscopy of protonated leu-enkephalin and its 18-crown-6 complex embedded in helium droplets. Chem. Eur. J. 2015, 21 (5), 2083-2089.
Hazard Statements: H302-H313-H319
Harmful if swallowed. May be harmful in contact with skin. Causes serious eye irritation.
Precautionary Statements: P280i-P264-P305+P351+P338-P301+P312a-P337+P313-P501a
Wear eye/face protection. Wash thoroughly after handling. IF IN EYES: Rinse cautiously with water for several minutes. Remove contact lenses, if present and easy to do. Continue rinsing. IF SWALLOWED: Call a POISON CENTER/doctor if you feel unwell. If eye irritation persists: Get medical advice/attention. Dispose of contents/container in accordance with local/regional/national/international regulations.